1. Field of the Invention
The present invention relates to an image reader, an image processor, and a computer program product.
2. Description of the Related Art
In a conventional image reader, when shading correction of eliminating irregularities in a lamp light amount distribution is carried out every time (i.e., reference white sheet data is also collected once every time each single sheet of an original document) is read in a continuous reading operation in a document feeder (DF) reading mode, reciprocation is performed between a reference white sheet and a DF reading position, which unnecessarily takes time. Therefore, the conventional image reader has a problem of a reduction in productivity.
Further, in the continuous reading operation in the DF reading mode by the conventional image reader, the following problem also occurs.
As shown in FIG. 18, when a color CCD (e.g., an RGB three-line CCD) fetches an image, a reference white sheet 101 is read while moving a carriage 100 in a forward direction (a document carrying direction) (it is assumed that data is fetched in the order of R, G, and B at the time). Since an original document 200 carried in the DF reading mode is carried with respect to the carriage 100 moved in a return direction (an opposite direction of the document carrying direction), data on the original document is read in the order of B, G, and R (this is opposite to the order of reading the reference white sheet 101). Furthermore, RGB data read by the color CCD is generally subjected to “interlinear correction” of increasing a delay amount of data read earlier to match positions of R, G, and B. That is, when the reading order of the RGB data in reading the reference white sheet 101 and the original document 200 is opposite, the order of data to be delayed by “interlinear correction” must be reversed. Therefore, a load is further increased with respect to a period where a CPU load is already large, thereby inhibiting an improvement in productivity.
Thus, Japanese Patent Application Laid-open No. 2002-247312 discloses a document reader that changes a time interval in intermittent shading according to a required image quality to improve productivity. Generally, reference white sheet data is collected just once every predetermined sheet of an original document, and an interval in the collecting operation varies depending on a required image quality, e.g., that in a photograph mode or a character mode.
Moreover, Japanese Patent Application Laid-open No. 2002-344708 discloses an image reader and an image former that move a carriage to a reference white sheet at a high velocity and read the reference white sheet during a reduction in speed, precipitously decrease a velocity of the carriage to shorten a time until return after reading, and slowly move the carriage in the return operation so as to facilitate convergence of vibrations of an optical system in a document reading operation. As a result, productivity of a continuous reading operation in the DF reading mode can be improved, and stable reference white sheet data can be collected.
However, according to the document reader disclosed in Japanese Patent Application Laid-open No. 2002-247312, an interval between collection of reference white sheet data, and subsequent collection of the same is set based on an elapsed time from lighting or a variation in a temperature, but a lamp (xenon lamp) has temperature characteristics. Therefore, an inconvenience may possibly occur. That is, a variation in luminescent characteristics differs depending on a situation where the lamp is turned on when it is warmed and a situation where the lamp is turned on when it is still cold. When a margin is given with respect to a variation in luminescent characteristics, productivity is sacrificed. When a margin is reduced, an image defect may occur.
Additionally, according to the image reader and the image former disclosed in Japanese Patent Application laid-open No. 2002-344708, when continuously reading many sheets of an original document in the DF reading mode, a CPU has many events to be processed in a period where a carriage is moved to a reference white sheet to acquire data, and a load of the processing is large. Further, before reading the reference white sheet, black offset data serving as a reference for image data must be obtained. When processing, e.g., acquisition of black offset data or obtainment of reference white sheet data is inserted into a period where a CPU load is large, the processing may not be terminated within a specified time.